1. Field of Invention
This invention relates generally to the removal of oil and grease from metal components (hereinafter referred to as degreasing). Specifically, this invention is an efficient and novel degreasing system which, in sequence, pyrolizes the oil and grease from metal components by indirect heating, combusts the environmentally harmful hydrocarbon-filled evaporated gases which are produced in the pyrolysis operation, and discharges the environmentally safe hydrocarbon-less gases which result from the combustion process.
The degreasing of metal components is necessary for a variety of operations and for a number of different components. For instance, automobile air conditioner evaporator fins are typically coated with a lubricating oil during their fabrication or formation. Subsequent to their formation, the fins must also be brazed. However, prior to the brazing step, the fins must be very clean and all lubricating oil must therefore be removed from the fins. Metal components must also be degreased during the metal reclamation process wherein all metal contaminants are removed from the components prior to melting of the metal.
2. Related Art
One method which is used to degrease metal components is aqueous cleaning, in which an already heated metal component is dipped in a chemically reactive solvent, such as trichloroethylene. The pre-heating and the chemical reaction which results with the solvent raise the temperature of the metal component to a temperature that is higher than both the vapor point of the oil and the vapor point of the solvent. Thus, when the metal component is removed from the solvent bath, all oil and solvent thereon evaporates leaving a solvent-less and oil-less metal component. Illustrative of such aqueous cleaning is U.S. Pat. No. 2,104,102 issued to Ruthven. Aqueous cleaning, however, requires the additional expense of the solvent, and, depending on the solvent and solvent-induced by-products, may also be environmentally hazardous.
Other degreasing methods which raise the temperature of metal components above the oil vapor point do so without regulating or limiting the heating temperature. Illustrative of such methods are U.S. Pat. No. 2,856,33 issued to Topelian and U.S. Pat. No. 4,684,411 issued to Johnson et al. An excessively high heating temperature, however, produces a thickening of the protective oxide layer of the component, particularly in components that are aluminum based. The thickening of the oxide layer is unacceptable for those components which must be subsequently brazed, such as automobile air conditioner fins.
In addition, many degreasing methods heat the metal components without maintaining a homogenous heating temperature. A non-homogenous heating temperature is particularly problematic in methods which utilize a furnace, oven, or rotary kiln to heat the components. Non-homogenous heating of the metal components may in turn result in incomplete evaporation of the oil from the metal components. U.S. Pat. No. 4,548,651 issued to Ramsey shows the use of a rotary kiln.
Pyrolysis is another degreasing method wherein the metal components are exposed to gases or air having a temperature higher than the vapor point of the oil. In pyrolysis, the metal components can either be directly or indirectly heated. Direct heating occurs if the metal components are exposed to gases or air which were heated by direct combustion and thus include the products of combustion while surrounding the components. Illustrative of such direct heating are U.S. Pat. No. 3,627,289 issued to Erman and U.S. Pat. No. 4,201,370 issued to Evans et al. Indirect heating, on the other hand, occurs if the metal components are exposed to gases or air which do not include the products of combustion. The use of direct heating results in the production of water vapor as the oil is evaporated from the metal components thereby creating a likelihood for condensate to form not only inside of the heating system, but also on the metal components. Such condensate formation is highly disfavored, particularly for components that must be subsequently brazed.
Also of concern in degreasing operations is the generation of environmentally harmful by-products. For example, the pyrolysis operation generates environmentally harmful hydrocarbon-filled gases. These environmentally harmful by-products must somehow be safely disposed or treated prior to disposal.
Necessarily, residence heating time is also an important parameter of degreasing operations. The residence heating time for any given degreasing operation must be long enough to ensure that all oil has been removed from the metal component, but must be short enough to make the operation and the turnover of metal components efficient. A shorter residence heating time is always preferred provided that the components are adequately cleaned.
U.S. Pat. No. 5,016,809 issued to Winterbottom et al. teaches a process to degrease aluminum based sheets essentially comprising the heating of the sheets in a reactive atmosphere at a temperature between 300 and 400.degree. C. for about 10 minutes to about 30 minutes thereby evaporating the oil and grease from the sheets without concurrently thickening the oxide layer of the sheets. However, the Winterbottom Patent does not address the benefits of and needs for a homogenous heating temperature and a reduction of residence heating time, as addressed herein. Furthermore, although it teaches a pyrolysis operation, the Winterbottom Patent does not address the considerable and important distinction between utilizing indirect versus direct heating. Finally, the Winterbottom Patent does not discuss an adequate and environmentally safe disposal of the hydrocarbon-filled gases which are generated by the pyrolysis operation.